Hot, humid days ahead in Rockford

ROCKFORD — The hottest stretch of summer may hit the Rockford area this weekend.

Temperatures may feel as warm as 100 degrees when you account for humidity.

A hot and humid air mass is set to descend on the Rock River Valley, breaking an unseasonably cool summer that included the third coolest July on record in Rockford, according to the National Weather Service.

Highs of 90 degrees are in the forecast for Friday, Saturday and Sunday.

“If that occurred that would technically be the first heat wave of the year,” said Ricky Castro, a meteorologist with the National Weather Service in Romeoville.

The weather service defines a heat wave for this region as three consecutive days hotter than 90 degrees. Those highs are uncertain because of the potential thunderstorms also in the forecast for the next few days. But even if temperatures don’t hit 90, the humid air mass is expected to deliver a few hot and sticky days.

“The air mass will certainly be the hottest of summer overall,” Castro said. “The heat index is going to be very uncomfortable.”

Castro said the heat index value with the humidity may be close to 100 degrees for three straight days, whether or not the actual temperatures hit 90.

So far this summer 90-degree days have been seldom. There have been just two days with temperatures at or above 90, the most recent on July 22. There were 15 days last year when temperatures hit or topped 90 degrees and 49 in 2012, which tied for the third most 90-degree days on record. The record was 62 such days in 1921.

There are typically 15 to 20 days with temperatures at or above 90 degrees each summer, according to the National Weather Service. But you don’t have to go too far back to find a summer with sparse 90-degree temperatures. In 2009, which had the third-coolest summer on record, there were four 90-degree days. In 2008, there were just two 90-degree days all year, both coming in September, according to the weather service.